Although an egg comes with its particular natural defense system (i.e. the eggshell), bacterial contamination of egg contents can nevertheless occur. Commonly, there are two possible ways by which this may happen: within the oviduct before the shell is formed, or through penetration of damaged and inferior quality shells. Whilst both routes are possible, the latter is the more probable. Supposing that unidentified but contaminated eggs find their way into the market place, this could become health hazard for consumers. Moreover, eggs with damaged shells will not be purchased by consumers and this can lead to considerable economic losses. The image of the egg sector strongly suffers from incidences of this kind and this is why breeding companies are looking for alternative selection focuses such as improved egg quality, of which shell quality is of considerable significance.
Commonly, shell quality is used as a synonym for shell strength and defines the capacity of eggshells to endure externally applied loads without cracking or breaking. Shell strength has only moderate heritability, and causes a problem because deteriorating eggshell quality is predominantly expressed late in the laying period. Commercial breeding companies have incorporated eggshell strength in their selection programs for many years. Selection for shell strength is being practiced by primary poultry breeders, using a variety of destructive and non-destructive methods for determining optimal selection variables. The latter have the advantage that the eggs can still be used after measurement, but in view of the low price per egg and EU food safety regulations, this argument carries less weight than speed and accuracy of measurement, heritability and genetic correlation with shell damage under commercial conditions.
A great deal of effort has gone into the design of tests that measure eggshell strength and these have formed the subject matter for several extensive reviews and prior art technology. The most commonly used methods to measure eggshell strength include specific gravity, shell deformation, shell thickness, shell percentage, structural properties, breaking strength, static stiffness and also a more recent method based on acoustic resonance frequency analysis for determining a non-destructive selection variable called dynamic stiffness.
In previous reports on the subject of eggshell strength it has been stated that the average egg breakage from point of lay to consumers' use was around 7%. However, despite the use of selection variables like the ones previously named, recent estimates indicate that this value has changed very little and thus, up to now, there is apparently no ideal variable for the genetic selection of laying hens for improved eggshell strength.
In EP738888 for instance a detector for determining cracks in eggshells is described. With this detector, the sound signal produced by a small ball briefly bouncing on a surface area of an egg is measured. More particularly, the curve of the sound intensity oscillating over time of the bouncing of the ball provides information about whether or not this surface area is intact. By carrying out this determination several times for the same egg, the condition of a shell of an egg, i.e. the presence or absence of cracks or ruptures in the eggshell, is mapped in an automated manner whereby a value for this condition is generated. Such a value is used as a criterion in the sorting of eggs. However, an important problem relates to the attachment of the measuring means to an egg. Such an attachment is quite difficult to manufacture. In addition, the detector disclosed in EP′ 888 is not easily applied to a batch of eggs and will take a long time to process and sort the eggs.
Therefore, there is a need of a novel approach for the determination of an indicator of eggshell strength.